1. Field of the Invention
The present invention relates to a liquid crystal display device (LCD), and more particularly, to an LCD and method for driving the same that are capable of achieving low power consumption and a high contrast ratio.
2. Discussion of the Related Art
In general, an LCD includes a liquid crystal layer interposed between two substrates, and there are two electrodes formed on the respective substrates. The LCD can display a predetermined image by aligning molecules of the liquid crystal layer using an electric field generated by applying a voltage to the two electrodes to adjust light transmittance varied with the alignment of the liquid crystal molecules. The LCD is a passive-type display using light from the outside, not spontaneously generating light. Therefore, the LCD is provided with a backlight unit as a light source for generating light from the outside. The backlight unit may be either an edge type or a direct type depending on the installation position of a lamp thereof.
The edge type backlight unit has a lamp installed around a lateral surface of a light guide plate. The edge type backlight unit is suitable for a small-sized LCD, such as a monitor of a laptop computer, a monitor of a desktop computer or the like. The edge type backlight unit has advantages in light uniformity and durability as well as manufacturing a slim profile LCD.
The direct type backlight unit is suitable for a large-sized LCD requiring high brightness because it has a higher light efficiency than the edge type backlight unit. In the direct type backlight unit, there are a plurality of lamps arranged in one column on a lower surface of a diffusing plate, and light is directly illuminated toward the front surface of a liquid crystal panel.
FIG. 1 is an exploded perspective view illustrating a direct type backlight unit according to the related art. Referring to FIG. 1, the related art direct type backlight unit includes a plurality of fluorescent lamps 1 having phosphors coated on inner surfaces thereof, a cover bottom 3 supporting and fixing the florescent lamps 1, and optical sheets 5a, 5b, and 5c disposed between the fluorescent lamps 1 and a liquid crystal panel (not shown). The optical sheets 5a, 5b, and 5c are designed to prevent the shapes of the fluorescent lamps 1 from being displayed on the liquid crystal panel and to supply light with a uniform brightness distribution on the whole. The optical sheets 5a, 5b and 5c may represent a diffusion sheet, a prism sheet, and a protection sheet, respectively, so as to enhance light scattering effect. A reflector 7 for reflecting light generated from the fluorescent lamps 1 to the liquid crystal panel is disposed inside the cover bottom 3 so as to enhance the efficiency of using light. Electrode connection lines 9a and 9b to which power is applied are provided on electrodes (not shown) at both ends of the fluorescent lamps 1.
The above-configured backlight unit generates constant brightness regardless of what kind of an image is displayed on the liquid crystal panel. However, the brightness fluctuates even in an image within one frame. Therefore, when a bright image is lightened more or a dark image is darkened more, a contrast ratio should be increased. Since the related art backlight unit always generates the constant brightness regardless of the characteristics of the image, the contrast ratio is deteriorated. In particular, since light with the constant brightness is applied regardless of an image's brightness in the related art, this causes the increase of power consumption. Moreover, it is necessary that an image needing a dark brightness should be displayed by light of a dark brightness and an image needing a bright brightness should be displayed by light of a bright brightness. Accordingly, an apparatus and method for controlling the light brightness according to an image's brightness are in great demand.